This invention relates generally to an apparatus and method for testing geophones during assembly. More particularly, this invention concerns an apparatus and method for determining the position of a coil within a geophone and the resonant frequency of the geophone during assembly.
Geophones, used in seismic exploration, are low frequency electro-magnetic devices which sense motion by means of suspending an inertial mass structure from a rigid, fixed reference supporting structure. The mass is a coil suspended by springs about a magnet and associated pole pieces, one spring attached to each end of the coil. The effect of the springs is to position the coil within the magnetic field and to give the "suspension system" a predetermined resonant frequency. As a geophone senses motion, a voltage signal results from the relative motion of the coil to the magnetic field. The geophone's accuracy is dependent upon the coil position relative to the magnet and associated pole pieces. The coil must be centered to assure linear travel through the magnetic flux, cutting the lines of force at an even rate to minimize distortion of the voltage waveform generated. The geophone's effectiveness is further dependent upon its resonant frequency as oscillation at its natural resonant frequency will produce the greatest signal output by the geophone.
A generally recognized testing method for determining whether the spring combination places the coil in a centered position is the use of a shadowgraph. The shadowgraph includes a holding fixture, a light source which is directed across the bottom of the geophone coil assembly and through a magnifying lens, and a calibrated screen. The coil form is projected onto the screen, with the position of the shadow being indicative of the coil position. This is strictly an optical method. One of the inherent problems associated with this method is that it is not adaptable to assembly line testing because of its size. In addition, there is a significant problem with respect to the accuracy of this device due to the amplification of the image of the geophone required.
The inability to test for the resonant frequency of a geophone is a substantial limitation of the shadowgraph method. If it is found that the natural frequency or harmonic distortion of the geophone tested after assembly is not within acceptable limits, the geophone must be disassembled, the spring combination changed, and the geophone must be reassembled and tested. This results in both loss of time and material.
The problems enumerated in the foregoing are not intended to be exhaustive, but rather are among several which tend to impair the effectiveness of existing methods and devices for testing geophones. Recognizing the need for an improved method and apparatus for testing geophones, it is therefore a general object of the present invention to provide a means of determining rapidly and reliably the position of the coil at rest and the resonant frequency of the geophone during assembly.